---
title: Release notes
description: "Zod 4 release notes and new features including performance improvements and breaking changes"
---

import { Callout } from "fumadocs-ui/components/callout";
import { Tabs, Tab } from "fumadocs-ui/components/tabs";
import { Accordion, Accordions } from 'fumadocs-ui/components/accordion';

After a year of active development: Zod 4 is now stable! It's faster, slimmer, more `tsc`-efficient, and implements some long-requested features. 

<Callout icon="❤️"> 
Huge thanks to [Clerk](https://go.clerk.com/zod-clerk), who supported my work on Zod 4 through their extremely generous [OSS Fellowship](https://clerk.com/blog/zod-fellowship). They were an amazing partner throughout the (much longer than anticipated!) development process.
</Callout>

## Versioning 

{/* <Callout> 
  **Update** — `zod@4.0.0` has now been published to npm. To upgrad
</Callout> */}

{/* To simplify the migration process both for users and Zod's ecosystem of associated libraries, Zod 4 will initially published alongside Zod 3 as part of the `zod@3.25` release. Despite the version number, it is considered stable and ready for production use. */}

To upgrade: 

```
npm install zod@^4.0.0
```


{/* Down the road, when there's broad support for Zod 4, we'll publish `zod@4.0.0` on npm. At this point, Zod 4 will be exported from the package root (`"zod"`). The `"zod/v4"` subpath will remain available. For a detailed writeup on the reasons for this versioning scheme, refer to [this issue](https://github.com/colinhacks/zod/issues/4371).  */}

For a complete list of breaking changes, refer to the [Migration guide](/v4/changelog). This post focuses on new features & enhancements.

{/* A number of popular ecosystem packages have Zod 4 support ready or nearly ready. Track the following pull requests for updates:
- [`drizzle-zod#4478`](https://github.com/drizzle-team/drizzle-orm/pull/4478)
- [`@hono/zod-validator#1173`](https://github.com/honojs/middleware/pull/1173) */}

## Why a new major version?

Zod v3.0 was released in May 2021 (!). Back then Zod had 2700 stars on GitHub and 600k weekly downloads. Today it has 37.8k stars and 31M weekly downloads (up from 23M when the beta came out 6 weeks ago!). After 24 minor versions, the Zod 3 codebase had hit a ceiling; the most commonly requested features and improvements require breaking changes.

Zod 4 fixes a number of long-standing design limitations of Zod 3 in one fell swoop, paving the way for several long-requested features and a huge leap in performance. It closes 9 of Zod's [10 most upvoted open issues](https://github.com/colinhacks/zod/issues?q=is%3Aissue%20state%3Aopen%20sort%3Areactions-%2B1-desc). With luck, it will serve as the new foundation for many more years to come.

For a scannable breakdown of what's new, see the table of contents. Click on any item to jump to that section.

## Benchmarks

You can run these benchmarks yourself in the Zod repo:

```sh 
$ git clone git@github.com:colinhacks/zod.git
$ cd zod
$ git switch v4
$ pnpm install
```

Then to run a particular benchmark:

```sh
$ pnpm bench <name>
```

### 14x faster string parsing

```sh
$ pnpm bench string
runtime: node v22.13.0 (arm64-darwin)

benchmark      time (avg)             (min … max)       p75       p99      p999
------------------------------------------------- -----------------------------
• z.string().parse
------------------------------------------------- -----------------------------
zod3          363 µs/iter       (338 µs … 683 µs)    351 µs    467 µs    572 µs
zod4       24'674 ns/iter    (21'083 ns … 235 µs) 24'209 ns 76'125 ns    120 µs

summary for z.string().parse
  zod4
   14.71x faster than zod3
```

### 7x faster array parsing

```sh
$ pnpm bench array
runtime: node v22.13.0 (arm64-darwin)

benchmark      time (avg)             (min … max)       p75       p99      p999
------------------------------------------------- -----------------------------
• z.array() parsing
------------------------------------------------- -----------------------------
zod3          147 µs/iter       (137 µs … 767 µs)    140 µs    246 µs    520 µs
zod4       19'817 ns/iter    (18'125 ns … 436 µs) 19'125 ns 44'500 ns    137 µs

summary for z.array() parsing
  zod4
   7.43x faster than zod3
```

### 6.5x faster object parsing

This runs the [Moltar validation library benchmark](https://moltar.github.io/typescript-runtime-type-benchmarks/). 

```sh
$ pnpm bench object-moltar
benchmark      time (avg)             (min … max)       p75       p99      p999
------------------------------------------------- -----------------------------
• z.object() safeParse
------------------------------------------------- -----------------------------
zod3          805 µs/iter     (771 µs … 2'802 µs)    804 µs    928 µs  2'802 µs
zod4          124 µs/iter     (118 µs … 1'236 µs)    119 µs    231 µs    329 µs

summary for z.object() safeParse
  zod4
   6.5x faster than zod3
```

## 100x reduction in `tsc` instantiations

Consider the following simple file:

```ts
import * as z from "zod";

export const A = z.object({
  a: z.string(),
  b: z.string(),
  c: z.string(),
  d: z.string(),
  e: z.string(),
});

export const B = A.extend({
  f: z.string(),
  g: z.string(),
  h: z.string(),
});
```

Compiling this file with `tsc --extendedDiagnostics` using `"zod/v3"` results in >25000 type instantiations. With `"zod/v4"` it only results in ~175. 

<Callout>

The Zod repo contains a `tsc` benchmarking playground. Try this for yourself using the compiler benchmarks in `packages/tsc`. The exact numbers may change as the implementation evolves. 

```sh
$ cd packages/tsc
$ pnpm bench object-with-extend
```
</Callout>

More importantly, Zod 4 has redesigned and simplified the generics of `ZodObject` and other schema classes to avoid some pernicious "instantiation explosions". For instance, chaining `.extend()` and `.omit()` repeatedly—something that previously caused compiler issues:

```ts
import * as z from "zod";

export const a = z.object({
  a: z.string(),
  b: z.string(),
  c: z.string(),
});

export const b = a.omit({
  a: true,
  b: true,
  c: true,
});

export const c = b.extend({
  a: z.string(),
  b: z.string(),
  c: z.string(),
});

export const d = c.omit({
  a: true,
  b: true,
  c: true,
});

export const e = d.extend({
  a: z.string(),
  b: z.string(),
  c: z.string(),
});

export const f = e.omit({
  a: true,
  b: true,
  c: true,
});

export const g = f.extend({
  a: z.string(),
  b: z.string(),
  c: z.string(),
});

export const h = g.omit({
  a: true,
  b: true,
  c: true,
});

export const i = h.extend({
  a: z.string(),
  b: z.string(),
  c: z.string(),
});

export const j = i.omit({
  a: true,
  b: true,
  c: true,
});

export const k = j.extend({
  a: z.string(),
  b: z.string(),
  c: z.string(),
});

export const l = k.omit({
  a: true,
  b: true,
  c: true,
});

export const m = l.extend({
  a: z.string(),
  b: z.string(),
  c: z.string(),
});

export const n = m.omit({
  a: true,
  b: true,
  c: true,
});

export const o = n.extend({
  a: z.string(),
  b: z.string(),
  c: z.string(),
});

export const p = o.omit({
  a: true,
  b: true,
  c: true,
});

export const q = p.extend({
  a: z.string(),
  b: z.string(),
  c: z.string(),
});
```

In Zod 3, this took `4000ms` to compile; and adding additional calls to `.extend()` would trigger a "Possibly infinite" error. In Zod 4, this compiles in `400ms`, `10x` faster.

> Coupled with the upcoming [`tsgo`](https://github.com/microsoft/typescript-go) compiler, Zod 4's editor performance will scale to vastly larger schemas and codebases.

## 2x reduction in core bundle size

Consider the following simple script.

```ts
import * as z from "zod";

const schema = z.boolean();

schema.parse(true);
```

It's about as simple as it gets when it comes to validation. That's intentional; it's a good way to measure the *core bundle size*—the code that will end up in the bundle even in simple cases. We'll bundle this with `rollup` using both Zod 3 and Zod 4 and compare the final bundles. 

| Package | Bundle (gzip) |
|---------|--------------|
| Zod 3   | `12.47kb`      |
| Zod 4   | `5.36kb`       |

The core bundle is ~57% smaller in Zod 4 (2.3x). That's good! But we can do a lot better.

## Introducing Zod Mini

Zod's method-heavy API is fundamentally difficult to tree-shake. Even our simple `z.boolean()` script pulls in the implementations of a bunch of methods we didn't use, like `.optional()`, `.array()`, etc. Writing slimmer implementations can only get you so far. That's where Zod Mini comes in. 

```sh
npm install zod@^4.0.0
```

It's a Zod variant with a functional, tree-shakable API that corresponds one-to-one with `zod`. Where Zod uses methods, Zod Mini generally uses wrapper functions:

<Tabs groupId="lib" items={[ "Zod Mini", "Zod"]}>
<Tab value="Zod Mini">
```ts
import * as z from "zod/mini";

z.optional(z.string());

z.union([z.string(), z.number()]);

z.extend(z.object({ /* ... */ }), { age: z.number() });
```
</Tab>
<Tab value="Zod">
```ts
import * as z from "zod";

z.string().optional();

z.string().or(z.number());

z.object({ /* ... */ }).extend({ age: z.number() });
```
</Tab>
</Tabs>

Not all methods are gone! The parsing methods are identical in Zod and Zod Mini:

```ts
import * as z from "zod/mini";

z.string().parse("asdf");
z.string().safeParse("asdf");
await z.string().parseAsync("asdf");
await z.string().safeParseAsync("asdf");
```

There's also a general-purpose `.check()` method used to add refinements.

<Tabs groupId="lib" items={[ "Zod Mini", "Zod"]}>
<Tab value="Zod Mini">
```ts
import * as z from "zod/mini";

z.array(z.number()).check(
  z.minLength(5), 
  z.maxLength(10),
  z.refine(arr => arr.includes(5))
);
```
</Tab>
<Tab value="Zod">
```ts
import * as z from "zod";

z.array(z.number())
  .min(5)
  .max(10)
  .refine(arr => arr.includes(5));
```
</Tab>
</Tabs>

The following top-level refinements are available in Zod Mini. It should be fairly self-explanatory which Zod methods they correspond to.

```ts
import * as z from "zod/mini";

// custom checks
z.refine();

// first-class checks
z.lt(value);
z.lte(value); // alias: z.maximum()
z.gt(value);
z.gte(value); // alias: z.minimum()
z.positive();
z.negative();
z.nonpositive();
z.nonnegative();
z.multipleOf(value);
z.maxSize(value);
z.minSize(value);
z.size(value);
z.maxLength(value);
z.minLength(value);
z.length(value);
z.regex(regex);
z.lowercase();
z.uppercase();
z.includes(value);
z.startsWith(value);
z.endsWith(value);
z.property(key, schema); // for object schemas; check `input[key]` against `schema`
z.mime(value); // for file schemas (see below)

// overwrites (these *do not* change the inferred type!)
z.overwrite(value => newValue);
z.normalize();
z.trim();
z.toLowerCase();
z.toUpperCase();
```


This more functional API makes it easier for bundlers to tree-shake the APIs you don't use. While regular Zod is still recommended for the majority of use cases, any projects with uncommonly strict bundle size constraints should consider Zod Mini.  

### 6.6x reduction in core bundle size

Here's the script from above, updated to use `"zod/mini"` instead of `"zod"`.

```ts
import * as z from "zod/mini";

const schema = z.boolean();
schema.parse(false);
```

When we build this with `rollup`, the gzipped bundle size is `1.88kb`. That's an 85% (6.6x) reduction in core bundle size compared to `zod@3`.

| Package   | Bundle (gzip) |
|-----------|--------------|
| Zod 3     |  `12.47kb`      |
| Zod 4 (regular)     | `5.36kb`       |
| Zod 4 (mini) | `1.88kb`       |

Learn more on the dedicated [`zod/mini`](/packages/mini) docs page. Complete API details are mixed into existing documentation pages; code blocks contain separate tabs for `"Zod"` and `"Zod Mini"` wherever their APIs diverge.

## Metadata

Zod 4 introduces a new system for adding strongly-typed metadata to your schemas. Metadata isn't stored inside the schema itself; instead it's stored in a "schema registry" that associates a schema with some typed metadata. To create a registry with `z.registry()`:

```ts
import * as z from "zod";

const myRegistry = z.registry<{ title: string; description: string }>();
```

To add schemas to your registry: 

```ts
const emailSchema = z.string().email();

myRegistry.add(emailSchema, { title: "Email address", description: "..." });
myRegistry.get(emailSchema);
// => { title: "Email address", ... }
```

Alternatively, you can use the `.register()` method on a schema for convenience:

{/* > Unlike all other Zod methods, `.register()` is *not* immutable, it returns the original schema. */}

```ts
emailSchema.register(myRegistry, { title: "Email address", description: "..." })
// => returns emailSchema
```

### The global registry

Zod also exports a global registry `z.globalRegistry` that accepts some common JSON Schema-compatible metadata:

```ts
z.globalRegistry.add(z.string(), { 
  id: "email_address",
  title: "Email address",
  description: "Provide your email",
  examples: ["naomie@example.com"],
  extraKey: "Additional properties are also allowed"
});
```

### `.meta()`

To conveniently add a schema to `z.globalRegistry`, use the `.meta()` method.

{/* > Unlike `.register()`, `.meta()` *is* immutable; it returns a new instance (a clone of the original schema). */}

```ts
z.string().meta({ 
  id: "email_address",
  title: "Email address",
  description: "Provide your email",
  examples: ["naomie@example.com"],
  // ...
});
```

<Callout>
For compatibility with Zod 3, `.describe()` is still available, but `.meta()` is preferred.

```ts
z.string().describe("An email address");

// equivalent to
z.string().meta({ description: "An email address" });
```
</Callout>

## JSON Schema conversion

Zod 4 introduces first-party JSON Schema conversion via `z.toJSONSchema()`.

```ts
import * as z from "zod";

const mySchema = z.object({name: z.string(), points: z.number()});

z.toJSONSchema(mySchema);
// => {
//   type: "object",
//   properties: {
//     name: {type: "string"},
//     points: {type: "number"},
//   },
//   required: ["name", "points"],
// }
```

Any metadata in `z.globalRegistry` is automatically included in the JSON Schema output.

```ts
const mySchema = z.object({
  firstName: z.string().describe("Your first name"),
  lastName: z.string().meta({ title: "last_name" }),
  age: z.number().meta({ examples: [12, 99] }),
});

z.toJSONSchema(mySchema);
// => {
//   type: 'object',
//   properties: {
//     firstName: { type: 'string', description: 'Your first name' },
//     lastName: { type: 'string', title: 'last_name' },
//     age: { type: 'number', examples: [ 12, 99 ] }
//   },
//   required: [ 'firstName', 'lastName', 'age' ]
// }

```

Refer to the [JSON Schema docs](/json-schema) for information on customizing the generated JSON Schema.

## Recursive objects

This was an unexpected one. After years of trying to crack this problem, I finally [found a way](https://x.com/colinhacks/status/1919286275133378670) to properly infer recursive object types in Zod. To define a recursive type:

```ts
const Category = z.object({
  name: z.string(),
  get subcategories(){
    return z.array(Category)
  }
});

type Category = z.infer<typeof Category>;
// { name: string; subcategories: Category[] }
```

You can also represent *mutually recursive types*:

```ts
const User = z.object({
  email: z.email(),
  get posts(){
    return z.array(Post)
  }
});

const Post = z.object({
  title: z.string(),
  get author(){
    return User
  }
});
```

Unlike the Zod 3 pattern for recursive types, there's no type casting required. The resulting schemas are plain `ZodObject` instances and have the full set of methods available. 

```ts
Post.pick({ title: true })
Post.partial();
Post.extend({ publishDate: z.date() });
```

## File schemas

To validate `File` instances:

```ts
const fileSchema = z.file();

fileSchema.min(10_000); // minimum .size (bytes)
fileSchema.max(1_000_000); // maximum .size (bytes)
fileSchema.mime(["image/png"]); // MIME type
```

## Internationalization

Zod 4 introduces a new `locales` API for globally translating error messages into different languages. 

```ts
import * as z from "zod";

// configure English locale (default)
z.config(z.locales.en());
```

See the full list of supported locales in [Customizing errors](/error-customization#locales); this section is always updated with a list of supported languages as they become available.

## Error pretty-printing

The popularity of the [`zod-validation-error`](https://www.npmjs.com/package/zod-validation-error) package demonstrates that there's significant demand for an official API for pretty-printing errors. If you are using that package currently, by all means continue using it. 

Zod now implements a top-level `z.prettifyError` function for converting a `ZodError` to a user-friendly formatted string.

```ts
const myError = new z.ZodError([
  {
    code: 'unrecognized_keys',
    keys: [ 'extraField' ],
    path: [],
    message: 'Unrecognized key: "extraField"'
  },
  {
    expected: 'string',
    code: 'invalid_type',
    path: [ 'username' ],
    message: 'Invalid input: expected string, received number'
  },
  {
    origin: 'number',
    code: 'too_small',
    minimum: 0,
    inclusive: true,
    path: [ 'favoriteNumbers', 1 ],
    message: 'Too small: expected number to be >=0'
  }
]);

z.prettifyError(myError);
```

This returns the following pretty-printable multi-line string:

```ts
✖ Unrecognized key: "extraField"
✖ Invalid input: expected string, received number
  → at username
✖ Invalid input: expected number, received string
  → at favoriteNumbers[1]
```

Currently the formatting isn't configurable; this may change in the future.

## Top-level string formats

All "string formats" (email, etc.) have been promoted to top-level functions on the `z` module. This is both more concise and more tree-shakable. The method equivalents (`z.string().email()`, etc.) are still available but have been deprecated. They'll be removed in the next major version.

```ts
z.email();
z.uuidv4();
z.uuidv7();
z.uuidv8();
z.ipv4();
z.ipv6();
z.cidrv4();
z.cidrv6();
z.url();
z.e164();
z.base64();
z.base64url();
z.jwt();
z.lowercase();
z.iso.date();
z.iso.datetime();
z.iso.duration();
z.iso.time();
```

### Custom email regex

The `z.email()` API now supports a custom regular expression. There is no one canonical email regex; different applications may choose to be more or less strict. For convenience Zod exports some common ones.

```ts
// Zod's default email regex (Gmail rules)
// see colinhacks.com/essays/reasonable-email-regex
z.email(); // z.regexes.email

// the regex used by browsers to validate input[type=email] fields
// https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input/email
z.email({ pattern: z.regexes.html5Email });

// the classic emailregex.com regex (RFC 5322)
z.email({ pattern: z.regexes.rfc5322Email });

// a loose regex that allows Unicode (good for intl emails)
z.email({ pattern: z.regexes.unicodeEmail });
```

## Template literal types

Zod 4 implements `z.templateLiteral()`. Template literal types are perhaps the biggest feature of TypeScript's type system that wasn't previously representable.


```ts
const hello = z.templateLiteral(["hello, ", z.string()]);
// `hello, ${string}`

const cssUnits = z.enum(["px", "em", "rem", "%"]);
const css = z.templateLiteral([z.number(), cssUnits]);
// `${number}px` | `${number}em` | `${number}rem` | `${number}%`

const email = z.templateLiteral([
  z.string().min(1),
  "@",
  z.string().max(64),
]);
// `${string}@${string}` (the min/max refinements are enforced!)
```

Every Zod schema type that can be stringified stores an internal regex: strings, string formats like `z.email()`, numbers, boolean, bigint, enums, literals, undefined/optional, null/nullable, and other template literals. The `z.templateLiteral` constructor concatenates these into a super-regex, so things like string formats (`z.email()`) are properly enforced (but custom refinements are not!).

Read the [template literal docs](/api#template-literals) for more info.

## Number formats

New numeric "formats" have been added for representing fixed-width integer and float types. These return a `ZodNumber` instance with proper minimum/maximum constraints already added.

```ts
z.int();      // [Number.MIN_SAFE_INTEGER, Number.MAX_SAFE_INTEGER],
z.float32();  // [-3.4028234663852886e38, 3.4028234663852886e38]
z.float64();  // [-1.7976931348623157e308, 1.7976931348623157e308]
z.int32();    // [-2147483648, 2147483647]
z.uint32();   // [0, 4294967295]
```

Similarly the following `bigint` numeric formats have also been added. These integer types exceed what can be safely represented by a `number` in JavaScript, so these return a `ZodBigInt` instance with the proper minimum/maximum constraints already added.

```ts
z.int64();    // [-9223372036854775808n, 9223372036854775807n]
z.uint64();   // [0n, 18446744073709551615n]
```

## Stringbool

The existing `z.coerce.boolean()` API is very simple: falsy values (`false`, `undefined`, `null`, `0`, `""`, `NaN` etc) become `false`, truthy values become `true`.

This is still a good API, and its behavior aligns with the other `z.coerce` APIs. But some users requested a more sophisticated "env-style" boolean coercion. To support this, Zod 4 introduces `z.stringbool()`:

```ts
const strbool = z.stringbool();

strbool.parse("true")         // => true
strbool.parse("1")            // => true
strbool.parse("yes")          // => true
strbool.parse("on")           // => true
strbool.parse("y")            // => true
strbool.parse("enabled")      // => true

strbool.parse("false");       // => false
strbool.parse("0");           // => false
strbool.parse("no");          // => false
strbool.parse("off");         // => false
strbool.parse("n");           // => false
strbool.parse("disabled");    // => false

strbool.parse(/* anything else */); // ZodError<[{ code: "invalid_value" }]>
```

To customize the truthy and falsy values:

```ts
z.stringbool({
  truthy: ["yes", "true"],
  falsy: ["no", "false"]
})
```

Refer to the [`z.stringbool()` docs](/api#stringbools) for more information.

## Simplified error customization

The majority of breaking changes in Zod 4 involve the *error customization* APIs. They were a bit of a mess in Zod 3; Zod 4 makes things significantly more elegant, to the point where I think it's worth highlighting here. 

Long story short, there is now a single, unified `error` parameter for customizing errors, replacing the following APIs:

Replace `message` with `error`. (The `message` parameter is still supported but deprecated.)

```diff
- z.string().min(5, { message: "Too short." });
+ z.string().min(5, { error: "Too short." });
```

Replace `invalid_type_error` and `required_error` with `error` (function syntax):

```diff
// Zod 3
- z.string({ 
-   required_error: "This field is required" 
-   invalid_type_error: "Not a string", 
- });

// Zod 4 
+ z.string({ error: (issue) => issue.input === undefined ? 
+  "This field is required" :
+  "Not a string" 
+ });
```

Replace `errorMap` with `error` (function syntax):

```diff
// Zod 3 
- z.string({
-   errorMap: (issue, ctx) => {
-     if (issue.code === "too_small") {
-       return { message: `Value must be >${issue.minimum}` };
-     }
-     return { message: ctx.defaultError };
-   },
- });

// Zod 4
+ z.string({
+   error: (issue) => {
+     if (issue.code === "too_small") {
+       return `Value must be >${issue.minimum}`
+     }
+   },
+ });
```

## Upgraded `z.discriminatedUnion()`

Discriminated unions now support a number of schema types not previously supported, including unions and pipes:

```ts
const MyResult = z.discriminatedUnion("status", [
  // simple literal
  z.object({ status: z.literal("aaa"), data: z.string() }),
  // union discriminator
  z.object({ status: z.union([z.literal("bbb"), z.literal("ccc")]) }),
  // pipe discriminator
  z.object({ status: z.literal("fail").transform(val => val.toUpperCase()) }),
]);
```

Perhaps most importantly, discriminated unions now *compose*—you can use one discriminated union as a member of another.

```ts
const BaseError = z.object({ status: z.literal("failed"), message: z.string() });

const MyResult = z.discriminatedUnion("status", [
  z.object({ status: z.literal("success"), data: z.string() }),
  z.discriminatedUnion("code", [
    BaseError.extend({ code: z.literal(400) }),
    BaseError.extend({ code: z.literal(401) }),
    BaseError.extend({ code: z.literal(500) })
  ])
]);
```

## Multiple values in `z.literal()`

The `z.literal()` API now optionally supports multiple values. 

```ts
const httpCodes = z.literal([ 200, 201, 202, 204, 206, 207, 208, 226 ]);

// previously in Zod 3:
const httpCodes = z.union([
  z.literal(200),
  z.literal(201),
  z.literal(202),
  z.literal(204),
  z.literal(206),
  z.literal(207),
  z.literal(208),
  z.literal(226)
]);
```

## Refinements live inside schemas

In Zod 3, they were stored in a `ZodEffects` class that wrapped the original schema. This was inconvenient, as it meant you couldn't interleave `.refine()` with other schema methods like `.min()`.

```ts
z.string()
  .refine(val => val.includes("@"))
  .min(5);
// ^ ❌ Property 'min' does not exist on type ZodEffects<ZodString, string, string>
```

In Zod 4, refinements are stored inside the schemas themselves, so the code above works as expected.

```ts
z.string()
  .refine(val => val.includes("@"))
  .min(5); // ✅
```

### `.overwrite()`

The `.transform()` method is extremely useful, but it has one major downside: the output type is no longer *introspectable* at runtime. The transform function is a black box that can return anything. This means (among other things) there's no sound way to convert the schema to JSON Schema.

```ts
const Squared = z.number().transform(val => val ** 2);
// => ZodPipe<ZodNumber, ZodTransform>
```

Zod 4 introduces a new `.overwrite()` method for representing transforms that *don't change the inferred type*. Unlike `.transform()`, this method returns an instance of the original class. The overwrite function is stored as a refinement, so it doesn't (and can't) modify the inferred type.

```ts
z.number().overwrite(val => val ** 2).max(100);
// => ZodNumber
```

> The existing `.trim()`, `.toLowerCase()` and `.toUpperCase()` methods have been reimplemented using `.overwrite()`.

## An extensible foundation: `zod/v4/core`

While this will not be relevant to the majority of Zod users, it's worth highlighting. The addition of Zod Mini necessitated the creation of a shared sub-package `zod/v4/core` which contains the core functionality shared between Zod and Zod Mini. 

I was resistant to this at first, but now I see it as one of Zod 4's most important features. It lets Zod level up from a simple library to a fast validation "substrate" that can be sprinkled into other libraries.

If you're building a schema library, refer to the implementations of Zod and Zod Mini to see how to build on top of the foundation `zod/v4/core` provides. Don't hesitate to get in touch in GitHub discussions or via [X](https://x.com/colinhacks)/[Bluesky](https://bsky.app/profile/colinhacks.com) for help or feedback.


## Wrapping up

I'm planning to write up a series of additional posts explaining the design process behind some major features like Zod Mini. I'll update this section as those get posted. 

For library authors, there is now a dedicated [For library authors](/library-authors) guide that describes the best practices for building on top of Zod. It answers common questions about how to support Zod 3 & Zod 4 (including Mini) simultaneously.

```sh
pnpm upgrade zod@latest
```

Happy parsing!<br/>
— Colin McDonnell [@colinhacks](https://x.com/colinhacks)
